P-n heterojunction structure of zinc oxide-based nanorod and semiconductor thin film, preparation thereof, and nano-device comprising same

ABSTRACT

A heterojunction structure composed of a p-type semiconductor thin film and n-type ZnO-based nanorods epitaxially grown thereon exhibits high luminescence efficiency property due to facilitated tunneling of electrons through the nano-sized junction and the use of ZnO having high exciton energy as a light emitting material, and thus it can be advantageously used in nano-devices such as LED, field effect transistor, photodetector, sensor, etc.

FIELD OF THE INVENTION

The present invention relates to a novel heterojunction structurecomprising a p-type semiconductor thin film and an n-type ZnO-basednanorod epitaxially grown thereon, which provides a light emittingdevice having improved luminescence properties.

BACKGROUND OF THE INVENTION

The Gallium nitride (GaN)-based blue light emitting diode (LED)developed by Nichia Chemical Co., Ltd. in 1993 uses a GaN p-n thin filmjunction to provide blue and green LED devices, and in 1997, a shortwavelength (404 nm) blue LED having a life span of about 10,000 hours atroom temperature has been developed using a nitride semiconductor.

Recently, an n-type oxide semiconductor, zinc oxide (ZnO), has attractedattention as an another efficient light emitting (λ=380 nm) material atroom temperature due to its interesting features: (1) a directtransition band structure, (2) a low power threshold for optical pumpingat room temperature and (3) a large exciton binding energy (60 meV).

However, the development of a p-n heterojunction structure of zinc oxidehas been hampered due to the difficulty of fabricating p-type ZnO.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea novel heterojunction structure formed by way of using n-ZnO in theform of a nanorod and a p-type semiconductor material other than p-ZnOin the form of a thin film, which can be advantageously used fornano-devices such as LED, field effect transistor, photodetector,sensor, etc.

It is another object of the present invention to provide a method forpreparing such a structure.

It is a further object of the present invention to provide a nano-deviceor an array thereof comprising such a structure.

In accordance with one aspect of the present invention, there isprovided a heterojunction structure comprising a p-type semiconductorthin film and an n-type ZnO-based nanorod epitaxially grown thereon.

In accordance with another aspect of the present invention, there isprovided a method for preparing said heterojunction structure, whichcomprises bringing the vapors of a Zn-containing metal organic compoundand an O₂-containing compound as reactants separately into contact witha p-type semiconductor thin film at a temperature in the range of 400 to700° C. under a pressure in the range of 0.1 to 10 torr to form a ZnOnanorod on the surface of the semiconductor film.

In accordance with a still another aspect of the present invention,there is provided a nano-device or an array thereof comprising saidheterojunction structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of the invention, whentaken in conjunction with the accompanying drawings, which respectivelyshow:

FIG. 1: a schematic diagram of the process for preparing a p-nheterojunction structure in accordance with Example 1 of the presentinvention;

FIG. 2: a schematic diagram of the light emitting diode devicecomprising a p-n heterojunction structure in accordance with the presentinvention;

FIGS. 3( a) and 3(b): scanning electron microscope scans of theZnO-based p-n heterojunction structures obtained in Examples 1 and 2 ofthe present invention, respectively; and

FIG. 4: the light emission spectrum of the LED obtained in Example 2 ofthe present invention, which comprises the heterojunction structureformed by epitaxially growing n-type ZnO nanorods on a p-type GaN thinfilm.

DETAILED DESCRIPTION OF THE INVENTION

The inventive heterojunction structure characteristically comprises ap-type semiconductor thin film and an n-type ZnO-based nanorodepitaxially grown thereon.

Also, a nano-device comprising said heterojunction structure can befabricated by forming electrodes using a thermal or electron beamevaporation technique on the opposing surfaces of the p-typesemiconductor thin film and n-type nanorods of the heterojunctionstructure.

In the inventive heterojunction structure, the p-type semiconductor thinfilm may be in the form of a single crystal, or a thin film formed on asubstrate using a conventional metal organic chemical vapor deposition(MOCVD) method which comprises heating a substrate and bringing thevapors of appropriate precursors into contact with the surface of thesubstrate.

The p-type semiconductor thin film of the inventive structure may have athickness ranging from 50 nm to 200 μm.

Suitable for use as the p-type semiconductor for a thin film is amaterial having a band-gap of 1.5 to 4.5 eV, and representative examplesthereof include a III-VB group semiconductor such as GaN, AlN, GaP,GaAs, etc.; a II-VIB group semiconductor such as ZnSe, CdSe, CdS, ZnS,etc.; and a semiconductor such as SrCu₂O₂, SiC, Si, etc.

Further, the ZnO-based nanorod grown on the p-type semiconductor thinfilm may be a ZnO nanorod, or a ZnO nanorod doped or coated with aheteromaterial. Exemplary heteromaterial dopants include Mg, Mn, Cd, Se,etc.; and doped heteromaterials, Zn_(1-x)Mg_(x)O (0<x<1),Zn_(1-x)Mn_(x)O (0<x<1), Zn_(1-x)Cd_(x)O (0<x<1), Zn_(1-x)Se_(x)O(0<x<1) and the like.

The inventive heterojunction structure can be prepared by epitaxiallygrowing ZnO-based nanorods onto a p-type semiconductor thin film using ametal organic chemical vapor deposition (MOCVD) method, which comprisesbrining the vapors of a Zn-containing metal organic compound and anO₂-containing compound as reactants separately into contact with ap-type semiconductor thin film at 400 to 700° C. under a pressure in therange of 0.1 to 10 torr.

If necessary, a ZnO nanorod formed on a p-type semiconductor thin filmmay be doped or coated with a heteromaterial by introducing the vapor ofa compound containing a heteromaterial such as Mg, Mn, Cd, Se, etc., atthe time of introducing the reactant vapors or after the formation ofthe ZnO nanorod, to form a heteromaterial-doped or coated ZnO nanorod.

The diameter, length and density of ZnO-based nanorods formed on ap-type semiconductor thin film can be varied depending on the reactionconditions such as the amount of gaseous reactants introduced into areaction chamber, deposition temperature and pressure, etc., duringtheir growth.

The nanorod on a thin film of the inventive structure may have adiameter in the range of 5 to 100 nm and a length in the range of 5 nmto 100 μm.

Exemplary Zn-containing metal-organic compounds that can be used asprecursors for zinc oxide in the present invention include dimethylzinc[Zn(CH₃)₂], diethylzinc [Zn(C₂H₅)₂], zinc acetate [Zn(OOCCH₃)₂. H₂O],zinc acetate anhydride [Zn(OOCCH₃)₂], zinc acetyl acetonate[Zn(C₅H₇O₂)₂], etc; O₂-containing compound, O₂, O₃, NO₂, H₂O (vapor),CO₂, C₄H₈O, etc.

Further, exemplary Mg-containing metal organic compounds that can beused as precursors for nanorod doping or coating in the presentinvention include bis(cyclopentadienyl) magnesium [(C₅H₅)₂Mg],bis(methylcyclopentadienyl) magnesium [(CH₃C₅H₄)₂Mg],bis(ethylcyclopentadienyl) magnesium [(C₂H₅C₅H₄)₂Mg],bis(pentamethylcyclopentadienyl) magnesium [{(CH₃)₅C₅}₂Mg], magnesiumacetate [Mg(OOCCH₃)₂. H₂O], magnesium acetate anhydride [Mg(OOCCH₃)₂],magnesium acetyl acetonate [Mg(C₅H₇O₂)₂. H₂O], etc; Mn-containing metalorganic compound, bis(cyclopentadienyl) manganese [(C₅H₅)₂Mn], etc;Cd-containing metal organic compound, diethylcadmium [(C₂H₅)₂Cd], etc;Se-containing metal organic compound, diethylselenium [(C₂H₅)₂Se], etc.

The inventive heterojunction structure composed of a p-typesemiconductor thin film and n-type ZnO-based nanorods vertically grownthereon can be used for an LED device as shown in FIG. 2.

The heterojunction structure according to the present invention uses ZnOhaving high exciton binding energy which can be advantageously used forLED having improved emission characteristics resulting from therecombination of excitons at room temperature.

Further, the inventive heterojunction structure may be a p-n nanojunction which facilitates electron tunneling to increase the lightemission area and comprises a nanorod having a high aspect ratio, andthus it can be used for light emitting devices having high luminescenceefficiency at room temperature or higher.

As ZnO-based nanorods are formed epitaxially on a thin film in theinventive heterojunction structure, an array of light emitting devicecomprising such a structure can be easily assembled to fabricate variousnanosystems.

The following Examples are intended to illustrate the present inventionmore specifically, without limiting the scope of the invention.

EXAMPLE 1 The Growth of ZnO Nanorods on a P-Type GaN Thin Film

ZnO-based p-n heterojunction structure was prepared by the process asshown in FIG. 1 as follows.

An Mg-doped GaN thin film was deposited on an Al₂O₃ substrate using aconventional MOCVD technique and annealed, to obtain a p-type GaN thinfilm having a thickness of 2 μm. The metal organic precursors used weretrimethylgallium (TMGa) and bis(cyclopentadienyl) magnesium ((C₅H₅)₂Mg);and the nitrogen precursor, NH₃.

Then, n-type ZnO nanorods were vertically grown on the p-type GaN thinfilm thus obtained, by injecting gaseous Zn(C₂H₅)₂ and O₂ at flow ratesin the ranges of 1 to 10 sccm and 20 to 100 sccm, respectively, with anargon (Ar) carrier gas and reacting the vapors for about 1 hour, toobtain a p-n heterojunction structure comprising n-ZnO nanorods grown onthe p-GaN thin film. The reactor pressure and temperature weremaintained in the ranges of 0.1 to 10 torr and 400 to 700° C.,respectively, during the ZnO nanorod growth.

A scanning electron microscope (SEM) photograph of the p-nheterojunction structure thus obtained is shown in FIG. 3( a), whichreveal ZnO nanorods having a 40 nm diameter and 1 μm length areuniformly and vertically grown on the surface of the GaN thin film.Further, an X-ray diffraction (XRD) study showed that the nanorods areepitaxially grown in the (0001) orientation on the GaN thin filmsubstrate having the same orientation.

EXAMPLE 2 Fabrication of a Light Emitting Diode

A light emitting diode was fabricated using the heterojunction structureprepared in Example 1 as follows.

First, the free space around the ZnO nanorods grown on a GaN thin filmwas filled up by depositing an insulating material (e.g., photoresist,polyimide, etc.) thereon, and then, the tip portion of the nanorods wasexposed by etching using a plasma. Subsequently, a Ti (10 nm)/Au (50 nm)top ohimic electrode was formed at the tip portion of the etched n-typenanorods; and a Pt (10 nm)/Au (50 nm) bottom electrode, on the p-typeGaN thin film, by a thermal or electron beam evaporation technique. Theapplied accelerating voltage and emission current were in the ranges of4 to 20 kV and 40 to 400 mA, respectively, during the electrodesdeposition, which was conducted under a reactor pressure of around 10⁻⁵mmHg, while keeping the substrate temperature at room temperature.

The cross-sectional morphology of the top electrode-formed ZnO nanorodswas investigated by scanning electron microscopy (SEM) and the result isshown in FIG. 3( b).

Also, a light emission spectrum of the LED thus obtained is shown inFIG. 4. The light emission was strong enough to be visually recognizableand its intensity did not decrease during a long period of repeatedoperation (several tens of cycles). Further, as shown in FIG. 4, thedevice has emission peaks at around 570 nm and 470 nm.

The above result suggests that the inventive heterojunction structure ofa p-type semiconductor thin film having epitaxially grown n-typeZnO-based nanorods has excellent light emission characteristics.

While the embodiments of the subject invention have been described andillustrated, it is obvious that various changes and modifications can bemade therein without departing from the spirit of the present inventionwhich should be limited only by the scope of the appended claims.

1. A nano-device comprising a p-n hetrojunction structure of a p-typesemiconductor thin film and an n-type ZnO-based nanorod epitaxiallygrown thereon, wherein free space around portions other than tipportions of the ZnO nanorod grown on the semiconductor thin film isfilled with an insulating material.
 2. The nano-device of claim 1,wherein the p-type semiconductor is made of a material having a band-gapenergy ranging from 1.5 to 4.5 eV.
 3. The nano-device of claim 2,wherein p-type semiconductor is made of a material selected from thegroup consisting of GaN, AlN, GaP, GaAs, ZnSe, CdSe, CdS, ZnS, SrCu₂O₂,and SiC.
 4. The nano-device of claim 1, wherein the p-type semiconductorthin film has a thickness ranging from 50 nm to 200 μm.
 5. Thenono-device of claim 1, wherein the ZnO-based nanorod has a diameter inthe range of 5 to 100 nm and a length in the range of 5 nm to 100 μm. 6.The nano-device of claim 1, wherein the ZnO-based nanorod is a ZnOnanorod or a heteromaterial-doped or coated ZnO-nanorod.
 7. Thenano-device of claim 6, wherein the heteromaterial is selected from thegroup consisting of Mg, Mn, Cd, Se and mixtures thereof.
 8. Thenano-device of claim 6, wherein the doped heteromaterial is selectedfrom the group consisting of Zn_(1-x)Mg_(x)O (0<x<1), Zn_(1-x)Mn_(x)O(0<x<1), Zn_(1-x)Cd_(x)O (0<x<1) and Zn_(1-x)Se_(x)O (0<x<1).
 9. Anano-system or an integrated circuit comprising the nano-device of claim1.